Many binary communication systems format data into packets, consisting of a plurality of symbols, in order to permit access to the available transmission medium bandwidth by multiple stations. Intensity modulation is easily generated and detected, and for this reason it is frequently employed to transmit binary data in optical fiber and wireless infrared (IR) communication systems,. Photoconductive-mode detection of optical signals by a photodiode may be modeled as a shot noise process, in which photons reach the photodiode at random intervals according to a Poisson distribution. Most of the photons generate photoelectron-hole pairs which produce a fluctuating current with a mean and variance proportional to the incident power and a substantially white spectral density.
The binary data signal may be encoded to eliminate the need for dc response in the system. Examples of appropriate encoding techniques that may be used for this purpose include pulse position modulation (PPM), of which the well-known Manchester code is a subset, Hedeman coding, delay modulation, and mBnB transformation.
An example of a high-performance receiver for binary-coded intensity modulated data is found in Eastmond, et al., "Receiver for Binary-Coded Wireless Optical Data", U.S. Pat. No. 5,355,242, Assignee: Motorola, Inc. In this receiver, the total gain is made sufficiently large so that a binary signal is produced at the receiver output by front-end noise even when a signal is not present. The presence of a continuous output from the receiver creates problems for some system protocols; for example, the CSMA/CD protocol of Ethernet cannot function if the channel appears to be continuously active. In this case, the receiver must include a squelch to mute the output when the signal is absent. Rapid squelch response is especially important in high-speed systems such as Ethernet, in which the duration of a bit is 100 microseconds and the duration of a packet may be substantially less than 1 millisecond.
In wireless optical systems, both the desired optical signal and ambient light sources, such as the sun and incandescent lighting, contribute to the photodiode current. Although the dc component of ambient light-induced photodiode current can be rejected from the receiver by a high-pass coupling network such as a transformer, the fluctuating component of this current at frequencies within the receiver passband may have sufficient amplitude to exceed the thermal noise floor. If the receiver noise floor increase is sufficient to exceed a predetermined squelch threshold, then a false indication of signal presence will be produced. Thus, a simple, fixed-threshold squelch is inadequate for a receiver which operates in the wireless environment.
Wireless receivers which include at least two receiving branches require that a figure of merit for each branch be developed, which will be used to maximize the received signal to noise ratio according to a predetermined strategy. An example of an appropriate strategy for a multiple-branch receiver in which the branches have non-overlapping fields of view is to mute the output from all branches except for the one producing the highest received signal strength indicator (RSSI) output.
Wireless optical systems are characterized by a propagation loss factor which varies as the inverse square of the distance separating the transmitter and receiver due to the spreading of energy in space. In order to accommodate the wide amplitude variations in both signal and noise encountered in the wireless environment, the RSSI should be proportional to the logarithm of the signal plus noise envelope.
A first example of prior art is the squelch used in optical fiber receivers. Since the fiber is a closed, guided-wave system, the photodiode produces no response to ambient light and the propagation loss is determined by fiber material, manufacturing, and installation imperfections. Consequently, it is common practice for an optical receiver squelch to employ an RSSI responding directly to the signal envelope, not to its logarithm, and to compare the received signal envelope to a predetermined threshold.
A second example of a high-speed data squelch known in the art is found in Alameh, et al., "Method and Apparatus for Detecting Binary Encoded Data", U.S. Pat. No. 5,490,175, Assignee: Motorola, Inc. Signal presence is determined by examining the zero crossing pattern of the receiver output, but the relative signal amplitude in two or more branches cannot be determined by this example of prior art as disclosed.
A third example of prior art is the logarithmic RSSI used in many cellular and land-mobile radio receiver IF amplifier/limiter ICs, such as the Motorola MC13158 described in Analog/Interface ICs Device Data, Vol. II, Series J, First Printing .COPYRGT.MOTOROLA, lNC., 1995. A fundamental limitation to the use of such logarithmic processing in a high-speed data squelch is the extended recovery time following a data packet. This behavior, which is evident in published experimental results, e.g., Hughes, R. S., Logarithmic Video Amplifiers, Artech House, Inc., .COPYRGT.1971, FIG. 83, can be better understood by considering the post-packet transient response of a logarithmic RSSI, f(t), given by ##EQU1## where A is the packet signal amplitude, and .tau. is the recovery time constant. This may be re-written as ##EQU2## Note that the logarithmic recovery transient is a linear, rather than an exponential function of time t.
If a voltage, log.sub.10 {A.sub.0 }, defines the RSSI level at which the squelch threshold is set, then the RSSI response will assume this level at a time t.sub.0, which represents the time delay between the cessation of a packet and muting of the receiver output. It can then be shown that ##EQU3##
Consider a wireless Ethernet receiver, consisting of a detector, baseband bandpass filter, and limiter/RSSI connected in tandem. The lower (high-pass) cutoff frequency of the baseband bandpass filter preceding the limiting amplifier, f.sub.L1, is 500 kHz, and the corresponding recovery time constant is ##EQU4## or 3.2 bits. If the packet envelope peak amplitude is 60 dB above the squelch threshold, then EQU t.sub.0 =(0.115).multidot.(320.multidot.10.sup.-9).multidot.(60)=2.2 .mu.s,
to or 22 bits, and a burst of noise having significant duration will follow the packet. The recovery time cannot be significantly reduced by increasing f.sub.L1 without incurring an unacceptable increase in signal distortion at the limiter output.
Accordingly, in a receiver for intensity modulated binary coded data packets there is need for a method and device to provide an accurate indication of signal presence and magnitude over a wide dynamic range of signal and noise which exhibits reduced recovery time and also incorporates means for selecting the strongest signal among a plurality of receivers.